1. Field of the Invention
The present invention relates to an active type of display panel using light-emitting elements such as organic electroluminescent elements, a display device using the display panel and a method for driving the display panel.
2. Description of the Related Art
Electroluminescence display devices (referred to as EL display devices hereinafter) mounted with a display panel employing organic electroluminescence elements (referred to simply as EL elements hereinafter) in the form of light emitting elements carrying pixels are currently attracting attention. Known systems for driving display panels by means of these EL display devices include simple matrix type and active matrix type systems. In comparison with simple matrix type systems, active matrix type EL display devices consume very little electrical power and afford advantages such as low cross-talk between pixels, and are particularly suitable as large screen display devices and high definition display devices, and so forth.
As shown in FIG. 1, EL display devices are constituted by a display panel 1, and a driving device 2 for driving the display panel 1 in accordance with an image signal.
The display panel 1 is formed having an anode power line 3, a cathode power line 4, m data lines (data electrodes) A1 to Am arranged in parallel so as to extend in the perpendicular (vertical) direction of one screen, and n horizontal scanning lines (scanning electrodes) B1 to Bn for one screen which are orthogonal to the data lines A1 to Am. A drive voltage Vc is applied to the anode power line 3 and a ground potential GND is applied to the cathode power line 4. Further, pixel portions E1.1 to Em.n each carrying one pixel are formed at the points of intersection between the data lines A1 to Am and the scanning lines B1 to Bn of the display panel 1.
The pixel portions E1.1 to Em.n have the same constitution and are constituted as shown in FIG. 2. That is, the scanning line B is connected to the gate G of a scanning line selection FET (Field Effect Transistors) 11, and the data line A is connected to the drain D thereof. The gate G of a FET 12, which is a light emission drive transistor, is connected to the source S of the FET 11. When the drive voltage Vc is applied via the anode power line 3 to the source S of the FET 12, a capacitor 13 is connected between this gate G and source S. In addition, the anode terminal of the EL element 15 is connected to the drain D of the FET 12. A ground potential GND is applied through the cathode power line 4 to the cathode terminal of the EL element 15.
The driving device 2 applies a scanning pulse sequentially and alternatively to the scanning lines B1 to Bn of the display panel 1. In addition, the driving device 2 generates, in sync with the application timing of the scanning pulse, pixel data pulses DP1 to DPm which are dependent on the input image signals corresponding to the horizontal scanning lines, and applies these pulses to the data lines A1 to Am respectively. The pixel data pulses DP each have a pulse voltage which is dependent on the luminance level indicated by the corresponding input image signal. The pixel portions which are connected on the scanning line B to which the scanning pulse is applied are the write targets of this pixel data. The FET 11 in a pixel portion E which is the write target of this pixel data assumes an on state in accordance with the scanning pulse such that the pixel data pulse DP supplied via the data line A is applied to the gate G and to the capacitor 13 of the FET 12. The FET 12 generates a light emission drive current which is dependent on the pulse voltage of this pixel data pulse DP and supplies this drive current to the EL element 15. In response to this light emission drive current, the EL element 15 emits light at a luminance which is dependent on the pulse voltage of the pixel data pulse DP. Meanwhile, the capacitor 13 is charged by the pulse voltage of the pixel data pulse DP. As a result of this recharging operation, a voltage that depends on the luminance level indicated by the input image signal is stored in the capacitor 13 and so-called pixel data writing is then executed. Here, when discharge from the pixel data write target takes place, the FET 11 enters an off state, and the supply of the pixel data pulse DP to the gate G of the FET 12 is halted. However, because the voltage stored in the capacitor 13 as described above is continuously applied to the gate G of the FET 12, the FET 12 continues to cause a light emission drive current to flow to the EL element 15.
The light emission luminance of the EL elements 15 of each of the pixel portions E1.1 to Em.n depends on the voltage which is stored in the capacitor 13 as described above according to the pulse voltage of the pixel data pulse DP. In other words, the voltage stored in the capacitor 13 is the gate voltage of the FET 12 and therefore the FET 12 causes a drive current (drain current Id) that is dependent on the gate-source voltage Vgs to flow to the EL element 15. The relationship between the gate-source voltage Vgs of the FET 12 and the drain current Id is as shown in FIG. 3, for example. The flow of drive current through the EL element 15, which current is at a level that is dependent on the level of the voltage stored in the capacitor 13, constitutes the light emission luminance that depends on the level of the voltage stored in the capacitor 13. Thus, the EL display device is capable of a gray level display.
In a drive transistor such as the FET 12, the characteristic for the relationship between the gate-source voltage Vgs and the drain current Id changes according to temperature changes and inconsistencies in the transistor itself. For example, in cases where characteristics (characteristics indicated by solid lines) deviate from the standard characteristic (broken line) as shown in FIG. 4, the respective drain currents Id are different for the same gate-source voltage Vgs, and therefore the EL element cannot be caused to emit light at the desired luminance.
A voltage change range for the gate-source voltage Vgs with respect to the luminance change range which is required for the gray level display is established beforehand. If the characteristic for the relationship between the gate-source voltage Vgs and the drain current Id is standard, the current change range of the drain current Id with respect to the voltage change range of the gate-source voltage Vgs is as shown in FIG. 5A. The current change range of the drain current Id shown in FIG. 5A is a range that corresponds to the luminance change range required for the gray level display. On the other hand, in cases where there is a change in the relationship characteristic, the current change range of the drain current Id with respect to the pre-established voltage change range of the gate-source voltage Vgs differs from the luminance change range required for the gray level display shown in FIG. 5A, as shown in FIGS. 5B and 5C. Therefore, when there is a variation in the drive current characteristic with respect to the input control voltage as a result of a drive transistor temperature variation and inconsistencies in the transistor itself, a correct gray level display is not possible.